In 2000/2001, Millar became the youngest serving Mayor in Wales, a record which he understands is as yet unbroken, when he served the township of Towyn and Kinmel Bay. He has also served as a member of Conwy County Borough Council, the North Wales Police Authority and the North Wales Fire and Rescue Service.

Millar is a former Shadow Minister for the Environment and Planning, a former Shadow Minister for Communities and Local Government and also a former Shadow Minister for Economy and Transport. He is a former member of the National Assembly's Sustainability Committee, and a former Chair of the National Assembly's Health, Well-being and Local Government Committee.

In November 2010 he was elected as Chair of the Assembly's Public Accounts Committee. Millar also Chairs the Cross Party Group on Faith, and the Cross Party Group on the Armed Forces and Cadets, both of which he is a founder member.